There is increasing interest in developing polymerization processes that can be predictably controlled to produce polymers having specifically desired structures. One of the means for achieving such results is through a process of living polymerization. Such a process provides a higher degree of control during the synthesis of polymers having predictably well-defined structures and properties as compared to polymers made by conventional polymerization processes.
Controlled radical polymerization processes such as RAFT (reversible addition fragmentation chain transfer) provide useful embodiments of living polymerization processes. RAFT processes using xanthate or dithiocarbamate chain transfer RAFT agents are disclosed in WO 99/31144. RAFT processes using dithioester or trithiocarbonate chain transfer agents are disclosed in WO 98/01478, WO 200500319, WO 2005000924 and WO 2005000923.
The polymers produced by RAFT processes have end groups derived from the chain transfer agents used in these processes. For RAFT-derived polymers using xanthate, dithiocarbamate, dithioester or trithiocarbonate chain transfer agents, each polymer chain will contain at least one end group comprising a xanthate, dithiocarbamate, dithioester or trithiocarbonate functional group. In some end-use applications of the RAFT-derived polymers, it may be desirable to remove these functional groups and replace them with hydrogen.
WO 02/090397 discloses a process for substituting a dithiocarbonylated or dithiophosphorylated function on the chain end of a living organic polymer with a hydrogen atom by contacting the polymer with a source of free radicals and an organic compound bearing a labile hydrogen atom.
WO2005000923, WO2005003192, U.S. patent application Ser. No. 10/407,405, now U.S. Pat. No. 7,012,119 and U.S. patent application Ser. No. 10/609,225 now U.S. Pat. No. 6,988,439 disclose several methods for removing the sulfur-containing portion of a RAFT chain transfer agent from the polymer terminal end.
There remains a need for a RAFT end-group removal process that can be carried out on the RAFT polymer without first changing solvents or isolating the polymer product. There is also a need for a RAFT end-group removal process that allows for easy isolation of the end-group free polymer.